Reluctance machines are well known in the art. In general, a reluctance machine is an electric machine in which torque is produced by the tendency of a movable part to move to a position where the inductance of an excited winding is maximized (i.e., the reluctance is minimized).
In one type of reluctance machine, the phase windings are energized at a controlled frequency. This type of reluctance machine is generally referred to as a synchronous reluctance machine. In another type of reluctance machine, circuitry is provided to determine the position of the machine's rotor, and the windings of a phase are energized as a function of rotor position. This type of reluctance machine is generally referred to as a switched reluctance machine. Although the description of the current invention is in the context of a switched reluctance machine, the present invention is applicable to all forms of reluctance machines, including synchronous and switched reluctance motors and to other machines that have phase winding arrangements similar to those of switched reluctance machines.
The general theory of design and operation of switched reluctance machines is well known and discussed, for example, in The Characteristics, Design and Applications of Switched Reluctance Motors and Drives, by Stephenson and Blake and presented at the PCIM '93 Conference and Exhibition at Nuremberg, Germany, Jun. 21-24, 1993.
Typical known reluctance machines include a stator, a rotor rotatably mounted with respect to the stator, where the stator defines a plurality of phase windings. In most known motors, each phase winding comprises a plurality of electrically conductive coils of wire (e.g., copper), and each coil of wire is wound about a different stator pole. FIG. 1 generally illustrates a conventional reluctance machine 10 having a rotor 12 defining four rotor poles and a stator 14 that defines six stator poles. For purposes of illustration, the stator poles are divided into three stator pole pairs 13A, 13B and 13C, with each pole pair comprising two opposing stator poles. Wound about the stator poles are three phase windings A, B and C where each phase winding comprises two coils and where the coils of phase winding A are wound around the poles of stator pole pair 13A, the coils of phase winding B are wound around the stator poles in group 13B, and the coils of phase C are wound around the stator poles in group 13C.
When one of the phase windings A, B and C is energized by establishing electric current in the phase winding, the stator poles associated with that phase winding (e.g., stator pole group 13A for phase A) will be excited and become electromagnets. In machine 10, as in most known conventional machines, each energizing coil is placed immediately adjacent the sides of the stator pole the coil is intended to excite. For example, coil 16 is wound about, and intended to excite, stator pole 15. Thus, the two side turn portions 16a and 16b of coil 16 are positioned immediately adjacent the sides of stator coil 15. In a similar manner, each of the other coils of machine 10 is positioned immediately adjacent to the stator pole it is intended to excite.
The basic mechanism for torque production in a reluctance motor is the tendency of the rotor to move into a position to increase the inductance and minimize the reluctance of the energized phase winding. This position of maximum inductance and minimum reluctance occurs when the pair of rotor poles are pulled into alignment with the excited pair of stator poles. In general, the magnitude of the torque produced by this mechanism corresponds to the magnitude of the current in the energized phase winding. For an ideal motor with no magnetic saturation, the instantaneous torque T is: ##EQU1## Where i is the instantaneous current in the energized phase winding and ##EQU2## is the derivative of the phase self inductance L with respect to the rotor position .theta.. In other words, the torque is proportional to the square of the current and to the angular rate of change of the phase self inductance. While all practical reluctance motors have some magnetic saturation this equation is useful for purposes of the present analysis.
Reluctance torque is developed in a reluctance machine by energizing a pair of stator poles when a pair of rotor poles is in a position of misalignment with the energized stator poles. The degree of misalignment between the stator poles and the rotor poles is called the phase angle. As the pair of rotor poles approach the aligned position, inductance increases until the rotor poles align with the excited pair of stator poles. This is the maximum inductance level. While the inductance is increasing, torque is positive, since ##EQU3## is positive. As the rotor pole rotates past the excited stator pole, inductance begins to decrease, making ##EQU4## negative, which means that a negative, or breaking torque is produced. To prevent this breaking effect on the rotor, at a certain phase angle in the rotation of the rotor poles to the position of maximum inductance, but before the position of maximum inductance is achieved, the current is removed from the phase, de-energizing the stator poles. Subsequently, or simultaneously, a second phase is energized. If the second phase is energized when the inductance between the second pair of stator poles and the rotor poles is increasing, positive torque is maintained and the rotation continues. Continuous rotation is developed by energizing and de-energizing the stator poles in this fashion. The total torque of an reluctance machine is the sum of the individual torques described above.
While conventional machines, like machine 10, have relatively high torque densities with respect to other forms of electric machines, the full potential of such machines is not fully realized. This is due to the above described pattern of energizing and de-energizing single stator pairs. In other words, in conventional machines, only one phase winding is energized at any given time. Thus, the entirety of the flux flow through the machine is steered through the stator poles that are excited by the energized phase winding.
FIG. 2 generally illustrates the main flow of flux that will be established when the phase A winding of machine 10 is energized. As illustrated, at such a time the coils surrounding stator poles 15 and 17 are energized such that stator poles 15 and 17 are excited the flux flow is "steered" through these excited poles. As FIG. 2 illustrates, the main flow of flux through machine 10 as energized in FIG. 2 is from excited stator pole 17, across the air-gap, through the rotor 12, across the air-gap, through stator pole 15 and through the stator back-iron (or yoke) back to excited stator pole 17. This main flux flow is reflected by the flux path 20a and 20b. Because this main flux path crosses through the air-gap and passes through the rotor, it tends to produce torque and thus provides a path for the main torque-producing flux.
From an analysis of FIG. 2, it may be noted that when only one phase winding is energized, only the excited stator poles are used for torque production. The unexcited stator poles adjacent the excited pole, e.g., poles 18a and 18b, are not used in torque production and are magnetically and electrically idle. Accordingly, in conventional machines like machine 10 of FIG. 2, at each instant of operation, there is a large amount of active material (e.g., stator iron) that is not fully utilized for torque production.
Further, since only one phase is energized at a time in a traditional switched reluctance machine, the torque developed by the machine is not smooth. Torque drops off steeply when the phase angle of the rotor is between the poles of the stator, when the inductance is minimized, then increases as the phase angle of the rotor moves toward alignment with a stator pole, when inductance is maximized. This rising and falling torque phenomenon is known as "torque ripple."
In addition to the problem of torque ripple, known reluctance machines often produce undesirable noise and vibration. As the inductance of a reluctance machine increases and decreases, the magnetic flux in parts of the machine changes in relation to the increasing and decreasing inductance. As a typical reluctance machine's pair of rotor poles moves into a position of alignment with a pair of energized stator poles, radial lines of magnetic flux deform the shape of the rotor and stator poles, decreasing the separation space between the poles. The changing flux results in fluctuating magnetic forces being applied to the ferromagnetic parts of the machine. These forces can produce unwanted noise and vibration. One major mechanism by which these forces can create noise is the ovalizing of the stator caused by magnetic forces normal to the air-gap. Generally, as the magnetic flux increases along a given diameter of the stator, the stator is pulled into an oval shape by the magnetic forces. As the magnetic flux decreases, the stator pulls or springs back to its undistorted shape. This ovalizing and springing back of the stator will produce audible noise and can cause unwanted vibration.
In the past the problem of torque ripple has sometimes been addressed by modifying the motor control circuitry. As one example, by profiling the current in a phase during the active time period when the phase is energized, the rate of change in the magnetic flux can be controlled resulting in less abrupt changes in machine torque. Prior art attempts at reducing noise in switched reluctance machines include using stiffer materials for stator construction and manufacturing to very precise specifications. These attempted solutions result in higher design, manufacturing, and maintenance costs, thereby detracting from one of the switched reluctance machine's primary benefits, economy.
The present invention is directed to overcoming, or at least reducing the effects of, the problems set forth above. Other features of the present invention will be apparent to those skilled in the art having the benefit of this disclosure.